This invention relates generally to demodulation techniques, and more particularly to demodulation in time division multiple access (TDMA) burst communication systems. In TDMA systems a number of independent data transmitting and receiving stations have access to a single data communication link on a time-division basis; that is, the available time for use of the link is allocated among the stations in some uniform manner, or the stations contend with each other for an allocation of time. TDMA burst systems are widely used in orbiting satellite communications. A single satellite may be responsible for receiving and re-transmitting messages from a large number of ground stations. In any event, the performance of a TDMA system depends to a large degree on the efficiency with which the functions of acquisition and demodulation are performed.
In many types of communication systems, information is transmitted over a communication medium, such as a cable, optical fiber or radio link, by means of a high-frequency carrier signal. At a transmitter, the carrier signal is coded or modulated in accordance with one of a number of modulation schemes, to encode the data for transmission. At a receiver, the modulated carrier signal is received and demodulated to recover the transmitted data.
Since the receiver may not have a signal source that is synchronized exactly with the received carrier signal, a first and necessary step prior to demodulation is "acquiring" the carrier signal, by synchronizing the phase or frequency of the received carrier with that of a local oscillator, to produce an exact match in frequency and phase angle. The local oscillator can then be used to effectively subtract out the carrier component and obtain the desired modulation signal.
For digital communications there is a further step in aquisition. Modulation of a carrier signal with digital data may be effected by any of a number of schemes. One of the most common is phase-shift-keying (PSK) modulation, in which digital information is encoded by means of phase shifts of the carrier signal. In quadrature PSK, the phase of the carrier may assume any of four values in the four quadrants, i.e. 45, 135, 225 or 315 degrees with respect to the reference phase angle of the unmodulated carrier. Phase changes representative of digital data are made to occur at the transmitter, but only in synchronism with a data clock signal. Since the data clock is not necessarily available at the receiver, the acquisition process includes the step of synchronizing the incoming data rate with a local clock signal. This step is sometimes referred to as bit synchronization. Once the carrier has been acquired and bit synchronization has been achieved, the data can then be recovered by demodulating the carrier signal.
It will be apparent that the steps of carrier acquisition and bit synchronization constitute unproductive time in the nature of necessary system overhead. In communication systems involving lengthy transmissions of large amounts of data, the overhead is relatively unimportant, but in other systems, such as TDMA burst systems, slow acquisition times translate into extremely low operating efficiencies. Another factor contributing to inefficiency is the degree of degradation that the demodulation system may impose on the theoretical bit error rate of the communication system. Any such degradation represents a reduction in the transmission data rate that is supportable by a given radiated power at the transmitter. Clearly, the ideal demodulation system for TDMA communications is one that has rapid acquisition times and performs close to the theoretical bit error rate limit.
Typically, a burst of information transmitted on a TDMA system includes a preamble preceding the actual data. The preamble comprises a first segment during which only the carrier is transmitted to permit carrier acquisition at the receiver, and a second segment during which a specific pattern of data is transmitted to permit bit synchronization. The carrier acquisition function in prior systems of this type has been implemented by a feedback control loop of some kind. Basically, the difference between the phase of the incoming carrier signal and the phase of a local reference oscillator is detected and used to adjust the phase of one of the signals until the measured difference is reduced to zero. In such feedback loops, the control signal representative of a correction in phase is proportional to the sine of the measured phase difference angle. For an initial phase difference close to 180 degrees, the correction control signal is close to zero, and acquisition is slow, since the small corrective signal takes some time to pull the phase angle away from its initial misalignment.
Another difficulty is that, since the carrier in a quadrature PSK modulation system is modulated by phase shifts of multiples of 90 degrees, the carrier tends to be suppressed or hidden by the modulation. Most prior systems seek to overcome this difficulty by some form of modulation-cross-modulation mixing, using techniques referred to as Costas loops or squaring loops, to recover a carrier signal for use during data modulation times. All of these techniques exhibit some amount of degradation due to the modulation and noise cross-products generated by mixing.
During that portion of the preamble during which a data pattern is received, usually 0, 1, 0, 1 . . . , bit synchronization must be performed. Most systems use a non-linearity, such as square law, to rectify the filtered data stream. The resulting waveform has a strong component at the data transition rate, which is then used to derive the bit synchronization clock signal. Traditionally, the bit synchronization apparatus has included a phase-locked feedback circuit to lock a local clock to the incoming bit rate. Unfortunately, this approach also suffers from long acquisition times if the initial alignment error is near 180 degrees.
It will be appreciated from the foregoing that there has been a need for an improved demodulation system suitable for use in TDMA burst communication systems. As already noted, the ideal demodulation system for this purpose should provide rapid carrier acquisition and rapid bit synchronization, without any degradation in bit transmission rate. The present invention fulfills this need.